Conventional heat exchangers used in motor vehicles typically comprise a core interposed between two header tanks. The core typically comprises multiple rows of hollow flat-sided tubes separated by, and in contact with, wave-shaped external cooling fins. The width of the tubes is thus substantially equal to the "depth" of the core, i.e. the distance from the front to the back of the core. The header tank typically comprises a manifold which is sealably secured to a header plate. The header plate has holes which are adapted to receive the ends of the tubes. The tubes are typically sealably secured to the header plates by soldering or brazing.
A fluid, either a liquid or air coolant, typically enters the heat exchanger through an inlet in the manifold of a first header tank. The fluid is then directed into the tubes where it radiates heat through the tube walls and cooling fins, which are in turn cooled by air flowing between the tubes. The fluid flows through the tubes into a second header tank where it is collected and directed through an outlet in the manifold of the second tank.
The tubes, fins and header tanks are typically made from metals such as aluminum, copper, brass or steel. When all components of the heat exchanger are made from aluminum, a high temperature brazing oven is required to sealably secure the tubes to the header plates and to secure the external cooling fins to the tubes. However, high temperature brazing ovens are expensive and therefore increase manufacturing costs. When the components of the radiator are made from copper and/or brass, the tubes and fins are soldered together and the tubes are soldered to the header plates to form a fluid-tight seal. When the radiator components are made from copper, for example, all the junctions between the various copper parts are precoated with solder or a solder tape is placed between the elements. The components are then clamped together and heated to provide soldered joints. One major disadvantage of heat exchangers having soldered or brazed seals is that such seals are prone to failure when subjected to repeated thermal or mechanical shocks. Thermal shocks may occur, for example, when an engine is started in cold weather and hot coolant flows suddenly into a cold radiator.
Some of the disadvantages of radiators having soldered or brazed seals have been overcome in the prior art by providing a joint sealed by a grommet between the tube and header plate. Such a construction is taught by U.S. Pat. Nos. 4,756,361; 5,205,354; and 5,226,235 to Lesage. These patents teach a system wherein tubes having a circular cross-section are sealably secured to a header plate provided with circular holes. Each hole in the header plate is provided with an individual resilient grommet having a circular bore which is adapted to receive and form a seal with the sides of the circular tube received in the hole. Heat exchangers having this construction have much better resistance to mechanical and thermal shocks than heat exchangers in which the tubes are soldered or brazed to the header plates. However, a primary disadvantage of the Lesage heat exchanger is that cooling efficiency is impaired, particularly where air is the coolant.
Because the tubes taught by the Lesage patents are circular and do not have flat sides, it is not possible to use conventional external cooling fins in the form of wave-shaped plates between the tubes and extending along a longitudinal axis defined by the length of the tubes. Instead, Lesage teaches cooling fins in the form of apertured plates which extend transversely to the longitudinal axis and which are provided with holes through which the tubes are inserted. A large number of these transverse fins must be provided for each radiator. The holes in the transverse fins have collars extending from one side of the fin to provide heat exchange contact between the tubes and each fin. After insertion through the fins, Lesage teaches that the tubes are mechanically expanded to provide a friction fit in the holes of the fins.
The transverse fins of Lesage must be punched with the holes for the tubes. This substantially increases manufacturing costs. On the other hand, conventional prior art fins comprising wave-shaped thin metal sheets do not need punching nor do they have to be manufactured with as high a degree of precision as the transverse fins taught by Lesage.
Conventional wave-shaped fins can be manufactured having a large number of undulations per unit length, thus increasing the surface area of the cooling fin and improving the efficiency of the heat exchanger. Furthermore, these conventional fins have a much greater area of contact with the sides of the tubes than the transverse fins taught by Lesage, thus increasing efficiency of heat transfer. In order to obtain the same efficiency, the heat exchanger of Lesage must be provided with a very large number of transverse cooling fins spaced a very small distance apart. The collars on the transverse fins of Lesage limit the number of transverse fins which may be provided on a given length of tube. Accordingly, conventional wave-shaped cooling fins can be more economical and efficient than the transverse fins taught by the Lesage patents.
In general, heat exchangers having flat-sided tubes and conventional wave-shaped external cooling fins are more efficient than the Lesage heat exchanger, particularly in cooling systems where air is the coolant. Flat sided tubes generally have a larger surface area than circular tubes and thus can provide more efficient heat transfer.
Further, the tubes of the Lesage heat exchanger core are arranged in a rectangular array rather than a single row. This leaves gaps between the tubes from the front to the back of the core, reducing cooling efficiency. In contrast, a core comprising a single row of flat-sided tubes provides a continuous cooling surface throughout the depth of the core. Also, the wave-shaped cooling fins between the flat-sided tubes are in continuous contact with the flat-sided tubes throughout the entire depth of the core.